Pulverized coal (PC) power plants currently produce over half the electricity used in the United States. In 2007, these plants emitted over 1900 million metric tons of carbon dioxide (CO2), and as such, accounted for 83% of the total CO2 emissions from electric power generating plants and 33% of the total US CO2 emissions. Eliminating, or even reducing, these emissions will be essential in any plan to reduce greenhouse gas emissions.
Separating CO2 from gas streams has been commercialized for decades in food production, natural gas sweetening, and other processes. Aqueous monoethanolamine (MEA) based solvent capture is currently considered to be the best commercially available technology to separate CO2 from exhaust gases, and is the benchmark against which future developments in this area will be evaluated. Unfortunately, such amine-based systems were not designed for processing the large volumes of flue gas produced by a PC plant. Scaling the MEA-based CO2 capture system to the size required for PC plants would result in an 83% increase in the overall cost of electricity for the PC plant. Applying this technology to all existing PC plants in the US would cost $125 billion per year, making MEA-based CO2 capture an unlikely choice for large-scale commercialization.
There are many properties that desirably would be exhibited, or enhanced, in any CO2 capture technology contemplated to be a feasible alternative to the currently utilized MEA-based systems. For example, any such technology would desirably exhibit a high net CO2 capacity, and could provide lower capital and operating costs (less material volume required to heat and cool, therefore less energy required). A lower heat of reaction would mean that less energy would be required to release the CO2 from the material. Desirably, the technology would not require a pre-capture gas compression so that a high net CO2 capacity could be achieved at low CO2 partial pressures, lowering the energy required for capture.
Moreover, CO2 capture technologies utilizing materials with lower viscosities would provide improved mass transfer, reducing the size of equipment needed, as well as a reduction in the cost of energy to run it. Low volatility and high thermal, chemical and hydrolytic stability of the material(s) employed could reduce the amount of material needing to be replenished, and could reduce the emission of degradation products. Of course, any such technology would also desirably have low material costs so that material make-up costs for the system would be minimized. Operability of CO2 release at high pressures could reduce the energy required for CO2 compression prior to sequestration. These technologies would also desirably exhibit reduced corrosivity to help reduce capital and maintenance costs, and further would not require significant cooling to achieve the desired net CO2 loading, reducing operating costs.
In some cases, it would be very desirable if the new CO2 capture technology could maintain reaction materials and reaction products in a liquid state on a relatively consistent basis. This would also allow better handling and transport of the materials through the CO2 capture systems, and could also contribute to lower operating costs.
Unfortunately, many of the above delineated desired properties interact and/or depend on one another, so that they cannot be varied independently; and trade-offs are required. For example, in order to have low volatility, the materials used in any such technology typically must have a fairly large molecular weight; but to have low viscosity, the materials must have a low molecular weight. To have a high CO2 capacity at low pressures, the overall heat of reaction needs to be high; but to have low regeneration energy, the overall heat of reaction needs to be low. Moreover, as of this time, it has been very difficult (if not impossible) to find CO2 capture materials that have relatively high CO2 absorbance capabilities, but that can also remain in a liquid state throughout the capture process.
Desirably, a CO2 capture technology would be provided that optimizes as many of the above desired properties as possible, yet without causing substantial detriment to other desired properties. At a minimum, in order to be commercially viable, such technology would desirably be low cost; and would utilize materials(s) having low volatility and high thermal stability. The materials should also have a high net capacity for CO2; and the capacity to remain in the liquid state during the CO2 capture process.